This project concerns the molecular mechanisms of syndromic and nonsyndromic hereditary hearing impairment. The research of this past year focused on characterization of the MITF (microphthalmia-associated transcription factor) gene which we had previously cloned from a human melanocyte cDNA library, using the mouse hanolog, mi, as a probe. The MITF gene encodes a transcription factor with the structure of basic- helix-loop-helix-zipper. Some transcription factors with basic-helix- loop-helix motif, like myoD, are known to direct cells to specific differentiation pathways. To test the possible involvement of MITF in cell differentiation, we transfected NIH/3T3 cells with MITF contained in an expression vector. Stable MITF transfectants showed morphological transformation. Morphologically transformed cells included dendritic cells which resembled melanocytes. As seen by Northern blotting, Western blotting and immunocytochemistry, MITF transfectants expressed some melanogenic markers such as tyrosinase and tyrosinase-related peptide 1. This leads us to believe that MITF is a critical gene for melanocyte differentiation. We have previously proposed that mutations in the MITF gene may be part of the pathogenesis of Waardenburg syndrome type 2, the symptoms of which may be explained by the depletion of melanocytes. This year our laboratory, as well as others, showed that this notion is correct. We have found mutations of the MITF gene in two families with WS2. We are currently analyzing the functional properties of mutant MITG genes we found which cause symptoms in affected members of the families. We are also trying to understand why WS2, while a result of mutations in MITF, is inherited in an autosomal dominant fashion.